1. Field of the Invention
This invention relates generally to voltage detection circuitry and, more particularly, to an improved means of providing a high impedance input for receiving a voltage detection signal suitable for use in an electronic smoke detector application.
2. Description of the Prior Art
Ionization smoke detectors are already well known in the art, and the operation of this type of smoke detector has been described in several publications including "Where There is Smoke . . . ", by Ronald K. Jurgen, IEEE Spectrum, August, 1976, and "Where there is Smoke, There Will Be An IC", by Bernard Cole, Electronics, May 12, 1977. A radioactive source within an ionization chamber causes air particles to become ionized. The ionized air will conduct a small electric current if a voltage is applied across the ends of the ionization chamber. Smoke particles within the ionization chamber will cause a decrease in the current flow which can be detected as a change in voltage by appropriate detection circuitry.
The current established within the ionization chamber is commonly in the range of picoamperes (10.sup.-12 amperes). Therefore, it is common to use a high impedance field-effect-transistor to sense the voltage across the ionization chamber in order to avoid loading the ionization chamber with excessive input currents. In several prior art circuits, the field-effect-transistor employed is a discrete transistor which may be connected between the ionization chamber and an integrated circuit used to detect changes in voltage. The addition of a discrete device causes the overall system cost to be increased.
Other prior art circuits have avoided the use of a discrete field-effect-transistor by fabricating the input field-effect-transistor within the integrated circuit detection chip. Although this approach decreases system cost, it has introduced the problem of leakage associated with the input of the integrated circuit itself and with the path used to couple the ionization chamber to the integrated circuit. Because the ionization chamber operates with such small currents, even small leakage currents will load the chamber and destroy its sensitivity.
Another problem generally encountered when the detection voltage signal is received directly by the integrated circuit chip, without the use of a discrete device, is that the detection input is susceptible to the destructive effects of large static voltages during handling and testing. It has been common in the art to protect the input of an integrated circuit containing IGFET (insulated-gate-field-effect-transistor) devices by connecting conventional diodes to the input to prevent static voltages from damaging the thin insulating layer between the gate and the channel of input transistors. However, the leakage normally associated with the protective diodes can ruin the sensitivity of the ionization chamber.
An approach suggested by one integrated circuit manufacturer is to leave the input unprotected and to surround the path leading from the ionization chamber to the integrated circuit with a leakage guard ring. The guard ring is biased with the fixed reference voltage used within the integrated circuit to compare against the voltage generated by the ionization chamber. However, this approach is unsatisfactory in that leakage currents can still be generated which impair the sensitivity of the ionization chamber, and in that no provision is made for protecting the input of the integrated circuit.